1. Field of the Invention
The present invention relates to a light source unit including a plurality of light emitting diodes (LED) for irradiating beam to an object. The invention relates also to a technique of controlling temperature of the light source area of such light source unit when the unit is employed in an apparatus, such as a film scanner, for obtaining image data.
2. Description of the Related Art
In the case of e.g. an image scanner section employed in a color copier for obtaining image data from an original document, according to the conventional image scanner section, an illuminating module using white LED's irradiates a beam on the document and the beam reflected from this document is then guided through mirrors, lenses to be impinged in focus on a 3-line sensor. This illuminating module includes a plurality of white LED's spaced apart with a predetermined distance therebetween and arranged linearly. A power supply system is provided for supplying power to the plurality of LED's from a PWM (pulse width modulation) control circuit. Further, a plurality of resistors acting as heat generating elements are provided adjacent the plural LED's. There is provided a further power supplying system for supplying power to the respective resistors from a different PWM control circuit than the above-mentioned PWM control circuit. These together make it possible to desirably set the light emission distribution of the LED in a main scanning direction and to desirably set also the heat generating amount of each of the plural resistors along the main scanning direction. In this conventional technique, there are also provided a plurality of temperature sensors disposed along the array of the LED's so that the plural resistors may be controlled respectively, based on result of detection by these temperature sensors. For this type of conventional technique, reference is made to e.g. Japanese Patent Application “Kokai” No.: Hei. 2002-281240 (paragraphs [0029]–[0033], paragraphs [0068]–[0072], and FIG. 10) (“Document 1” hereinafter).
A further type of conventional technique is known from Japanese Patent Application “Kokai” No.: Hei. 2001-45225 (paragraphs [0038]–[0051] and FIG. 3) (“Document 2” hereinafter), though this document does not describe control of light source temperature. This document discloses a film scanner including a light source comprising LED's and adapted for obtaining image data from a photographic film. In this, a plurality of LED chips for emitting rays of R (red), G (green), B (blue) and IR (infrared) are mounted in correspondence with each of the colors on an aluminum substrate. The rays from the LED's are guided via such optical components as a half mirror, a dichroic mirror, etc. onto the photographic film. The beam transmitting through the film is guided via a zooming lens to be impinged on an area-sensor type CCD sensor.
When a light source unit including LED's is employed in such scanner for obtaining color image, as described also in the afore-mentioned Document 1, i.e. Japanese Patent Application “Kokai” No.: Hei. 2002-281240, precision temperature control needs to be effected in order to maintain a predetermined amount of light from the respective LED's and also to maintain a predetermined light emission spectrum.
Then, in the case of the construction including a plurality of temperature sensors in correspondence with the LED's, in the technique of Document 1, the heat generating elements (resistors) are arranged in the vicinity of the LED's. In operation, the heat from each heat generating element is caused to affect each corresponding LED, thereby to maintain this LED at a target temperature. Therefore, for independent drive of each of the plural heat generating elements arranged along the array of the LED's (main scanning direction), the construction requires the PWM control circuit for controlling each heat generating element. This results in not only increased complexity of the circuit and physical enlargement of the entire apparatus, but also in cost increase due to the large number of components required. Especially if the heat from the heat generating element is caused to affect the LED, though not contemplated or described in Document 1, it will be needed to take into consideration such factors as radiation of heat from the heat generating element, conduction of heat through the atmosphere, conduction of heat from the heat generating element through the substrate on which the element is mounted. Then, when the heat generation amount from the heat generating element is adjusted, there will occur a certain delay in the effective transmission of this to the LED.
On the other hand, if other type of light source such as a halogen lamp, a xenon lamp is employed as the light source unit for use in obtaining color image data, this tends to invite some non-uniformity in the color temperature and/or heat generation. In order to avoid these inconveniences, Document 2 has confirmed the usefulness of the use of at least three kinds of LED's as the light source unit for irradiating the red, green and blue beams. In particular, in the case of an apparatus, such as a film scanner, which needs to obtain beam of each wavelength with high precision over a relatively wide range of visible light, it is desired that the apparatus can achieve no non-uniformity in the amounts of light and appropriate color temperature over the entire visible light range. However, the solution proposed by Document 2, i.e. Japanese Patent Application “Kokai” No.: Hei. 2001-45225, requires the heat generating elements for the temperature control and the PMW control circuits for controlling these elements respectively. Hence, it will be understood that this solution is not practical or insufficient in terms of desirable compactness of the entire apparatus and cost reduction.